Cell transplantation has shown promise in reducing neurological deficits associated with stroke. One of the most effective of these therapies is marrow stromal cells (MSCs), that has been demonstrated to be highly neurorestorative. In this application, we will investigate the mechanisms by which MSCs produce this neurorestorative effect. Our preliminary data strongly indicate that there is extensive axonal remodeling in both brain and spinal cord in response to MSC treatment which highly correlates with improvement of neurological function. Given these robust preliminary data, and the fact that the field of white matter changes after stroke is understudied, and that there are no investigations on the effect of cell-based therapies for stroke on spinal cord remodeling, we propose the following two specific aims: In Aim 1, we will test the effects of MSC treatment of stroke on axonal outgrowth in the brain and the spinal cord. We hypothesize that recovery of limb motor function after stroke depends on axonal remodeling of the corticospinal tract (CST). CST axons emanating from the ischemic boundary in the ipsilateral hemisphere and from the intact contralateral hemisphere sprout and extend to the denervated spinal neurons. MSCs enhance such axonal restructuring to promote functional recovery. In Aim 2, we will investigate the cellular and molecular mechanisms by which MSC treatment promotes neuronal remodeling after stroke. We hypothesize that neuronal remodeling in the central nervous system (CNS) after stroke with MSC treatment is mediated by astrocytes, the most numerous cells and the major endogenous repair mediators in the adult CNS. Neurite outgrowth after stroke is enhanced by MSC treatment via astrocytic increase in the net activity of tissue plasminogen activator (tPA) via modifying the balance of tPA/plasminogen activator inhibitor-1(PAI-1) level. In this application, we employ genetically modified CST-YFP mice in which the CST is specifically and completely labeled with yellow fluorescent protein (YFP) and tPA knockout (tPA-/-)mice, as well as an array of novel and well-established experimental techniques in our laboratory. To our knowledge, our work is the first to investigate tPA as amplifying neurite remodeling and thereby mediating the beneficial actions of exogenous cells in the CNS. This project elucidates the interaction between MSCs and parenchymal cells that lead to white matter changes in the brain and spinal cord by which the injured CNS can be remodeled. Our ultimate goal is to delineate the mechanistic underpinnings of cell-based therapy in the restorative treatment of stroke. The proposed studies have high translational significance and will advance the field of stroke recovery.